This invention relates to a metal plating process and apparatus which is shown with particular applicability to gold plating or flashing work loads such as electrical contacts.
It is desirable to gold plate electrical contacts for certain applications such as for use in telephone connectors, printed circuit boards, or the like, to provide a nontarnishing conductive finish on the contacts. The gold finish permits a simple wiping of such impurities as oxides from the contacts to clean the same and provide for quite sensitive mating or wiping surfaces for the contacts. Many such contacts are very small and fragile and may easily be damaged, such as by deformation during handling, which greatly increases the fail rate of the connectors with which the contacts are utilized. Furthermore, deformed contacts of this type cause a great deal of problems, such as machinery hangups and lost assembly time, when the contacts are assembled to the respective connectors with which they are to be used. One of the major areas resulting in deformation of the contacts during manufacture is during the gold plating process for the contacts. The contacts are normally carried in tumbler-type containers or barrels through various plating processes and the tumbling action of the contacts, in and of itself, often times results in damage to the contacts because the barrels desirably are rotated at a relatively high rate of rotation due to the actual plating operation of the process.
More particularly, during a plating operation, such as by pulse plating, controlled rotation of the contacts during the plating operation contributes to the production of uniformily and consistently plated work loads. As will be more apparent in the detailed description of the invention herein, it is desirable to control the work load rotation during the pulse-plating operation to a speed on the order of 9-10 rpm. However, this fast rate of rotation, while being very desirable during the pulse-plating operation, is undesirable while transporting the contacts within their tumbler type barrels through the air from station to station. Such a high rate of rotation results in a high rate of contact deformation.
Therefore, one of the principal features of the invention is to provide a process and apparatus for metal plating a work load, such as gold plating electrical contacts, wherein the work load is carried in a tumbler-type container or barrel from station to station along the process, and a differential speed of rotation is provided for the barrels and work loads so that the barrels rotate at a slower rate of rotation in air than in solution.
It also is important to control the various parameters of the pulse-plating operation in conjunction with controlling the rate of rotation of the work loads. These parameters include controlling the chemistry of the bath including the gold concentration within the tank at the pulse-plating station, as well as the duty cycle of the pulse-plating operation, the temperature of the chemical bath, and the like.
In accordance with the invention disclosed herein, the Baume of the chemical bath at the pulse-plating station is controlled on the order of 15.degree.-17.degree. specific gravity. The pH of the chemical bath is on the order of 3.7 or 3.8. The gold concentration of the chemical bath is on the order of 0.3 to 0.4 troy ounce gold per gallon of solution. The temperature of the chemical bath is on the order of 90.degree.-95.degree. F. The current density of the chemical bath at the pulse-plating station is on the order one ampere per square foot of work load to be plated. The duty cycle of the pulse-plating operation is on the order of one micro-second on and two micro-seconds off. As stated above, the rpm of the work load within the chemical bath is on the order of 9 or 10 rpm. By controlling these parameters, along with the distribution of the work load within the barrels, very uniform and consistent gold plating of the work load is obtained.
In the process of the present invention, various cascade-type rinse assemblies are utilized both before and after the gold plating station and other water and deionized water circulation means is provided and results in considerable savings in the total amount of solution required for the process. In addition, the gold residue in solution from a rinse tank or station following the gold plating station, is fed to a gold recovery station utilizing a vacuum distillation process. The distillate therefrom is returned to the solution in one of the cascade-type rinse tanks in the process, and the recovered gold is returned to the gold plating tank.
The electrical contacts, after they are rinsed following the gold plating operation, are dried in a low torque centrifugal dryer having a gradual start-up speed from zero to full rpm. Here again, the drying of electrical contacts of the character described herein, often times results in further contact deformation due to the high centrifugal forces involved. By using a low torque dryer which includes an eddy current control clutch drive motor, the contact deformation is greatly reduced.
Other objects, features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings.